1. Field of Invention
This invention relates to a Coriolis mass flowmeter whose stability, accuracy, and resistance to vibration are improved by enhanced isolation of internal vibration.
2. Description of the Prior Art
FIG. 1 shows a conventional mass flowmeter, such as disclosed in Japanese Unexamined Patent application Hei No. 6-109,512. The conventional flowmeter comprises a vibration tube 1 both ends of which are fixed to flanges 2. The flanges 2 are used to mount vibration tube 1 to a pipe A (not shown). An oscillator 3 is provided at the center of tube 1. Vibration sensors 4, 5 are provided on both sides of the center of vibration tube 1. The vibration tube 1 is fixed at both ends thereof in a housing 6. A measuring fluid is transmitted through tube 1 and oscillator 3 is caused to be driven by a means not shown.
Let the angular velocity of oscillator 3 in the direction of the vibration be [W ] and the flow velocity of the measuring fluid be [V], then the following Coriolis force is produced:
Fc=xe2x88x922 m[w]xc3x97[V], wherein m represents mass.
Thus, by measuring the amplitude of vibration which is proportional to the Coriolis force, mass flow can be measured. Note, that the symbols enclosed within the brackets [ ] represent vector quantities.
FIG. 2 shows another conventional flowmeter which uses a two-tube configuration vibration tube 1.
Although the vibration tube 1 is fixed at both ends thereof in a one straight tube Coriolis mass flowmeter, such as shown in FIG. 1, for limited size flowmeters, the end points cannot be completely fixed and hence vibrate slightly.
One cause of vibration is as follows. If vibration tube 1, through which a measuring fluid is passed, is deformed as shown in FIG. 3 (which shows an approximately first order mode resonance state) since the length of the tube becomes longer due to deformation, a tensile force is generated in the direction of the axis of the vibration tube. For example, if the center part of the tube 1 (which is a stainless steel tube of diameter 9.6 mm, wall thickness of 0.91 mm, and length of 400 mm) is deformed by 1 mm, a tensile force of 7.5 kgf is generated in the axial direction, as shown in FIG. 3.
If tube 1 is oscillated in the first order resonance mode, the tensile force becomes maximum at the maximum positive and negative deformation and becomes minimum (i.e. zero) at the locations where there is no deformation. The tensile force reaches both its maximum and minimum two times in an oscillatory period. That is, the tensile force in the axial direction of tube 1 is generated at twice the frequency of the excitation frequency. If there is leakage of vibration due to this tensile force, that is there is insufficient isolation of vibration, the following problems arise.
1. Since the Q value becomes lower, the internal vibration becomes unstable and the flow meter is likely to be affected by un-wanted vibration noise other than noise caused by excitation oscillation.
2. A large amount of energy is needed for excitation so that power consumption is increased.
3. The extent of vibration leakage varies greatly depending on environmental changes and/or external causes, such as installation method, piping stress, temperature, etc. Thus, the vibration state of the tube also varies and the zero point and span are likely to change.
That is, conventional Coriolis flowmeters are likely to be made unstable, less resistant to vibration, and less accurate by the fore-going and other environmental changes and/or external causes. The conventional flowmeter shown in FIG. 2 is not likely to leak vibrations when using the tuning fork principle illustrate in FIGS. 4 and 5. This is because the forces cancel out each other at the branching points when there are two vibration tubes 1 vibrating in the opposite directions to each other. However, a structure having one vibration tube without branching points would still have the foregoing problems.
An object of the invention is to overcome the aforementioned and other deficiencies, disadvantages and problems of the prior art.
Another object is to provide a Coriolis mass flowmeter which has improved stability, accuracy, and resistance to vibration by enhancing isolation of internal vibration.